Officials in the west are particularly concerned about potential meddling by Moscow in Germany, which goes to the polls in September.

Last month, Russia was forced to publicly deny that it was involved in an attempted coup during Montenegro’s election, after a Montenegrin prosecutor claimed “Russian state bodies” had played a role in an attempted putsch with the aim of stopping the Balkan country from joining Nato.

Speaking during a visit to Brussels with Johnson, Fallon said he hoped an investigation in Montenegro would establish the facts, adding that the UK’s policy over Russia was to “engage but beware”.

He said: “There is concern about Russian activity in the western Balkans. There is also concern, as you know, in Germany about potential Russian involvement in the German elections just as there has been in other European elections but, yes, the situation in the western Balkans remains extremely fragile.

“There is a growing body of evidence now that there has been a step change in Russian behaviour and Boris will leave them in no doubt as to our firmness on that.”

Talking to reporters in Brussels, Johnson said it was clear that Russia “was up to all sorts of no good”. He declined to respond when asked whether he would be meeting Vladimir Putin.

“I think it’s very important to understand that Russia has to be engaged on a twin-track basis,” he said.

“They are, I’m afraid, engaged in cyber-warfare, they’re engaged in undermining countries in the western Balkans – you saw what happened in Montenegro – to say nothing of Russia’s actions in Ukraine which are as everybody knows completely unacceptable.

“So point number one is to get that message across to the Russians. They’ve got to change, they’ve got to show that they can be trusted again and that I think that is very important.

“But it’s also important to stress that we are not in a new cold war with Russia. Neither in the UK, nor [from] our friends in the rest of the EU, nor in Washington, is there any appetite for a new cold war.”

Talking after a meeting of the EU’s foreign and defence ministers in Brussels, Fallon also said he was pleased Britain had successfully “resisted” a push by some members and the European commission to establish an EU military headquarters.

Kremlin rejects claims Russia had role in Montenegro coup plot

Read more

It has instead been agreed that the EU will have a small command centre, using existing buildings and staff, which will initially coordinate the three existing EU military training missions in Mali, Somalia and the Central African Republic. Fallon said that Britain’s position had been supported by “sensible middle of the road countries”.

He said: “You recall back in autumn there were proposals not just for an EU army but a new European headquarters. We have successfully resisted all of that this morning. There will be no new headquarters, no new general officer. The current director of the EU military staff will be director of the military planning and conduct capability.”

Asked whether Britain was merely delaying the inevitable in blocking the proposals, Fallon said the UK would continue to argue that it was dangerous to replicate Nato structures.

“We are not leaving the European Union when we deliver the article 50 letter. We are not going to be leaving the European Union any time soon. After we leave, it will of course be up to the member states themselves to work out the pace at which they want to integrate further.”

Fallon said the priority for EU members should be to increase defence spending to the 2% of GDP target. He said: “When some of the more ambitious plans were unveiled in Bratislava back in September, it fairly quickly became obvious that Britain was not alone in wanting to avoid duplication with Nato. There was strong support from the Balkan states, the Nordic states, from countries that I call sensible middle of the road countries like the Netherlands. So this is not just Britain.

“Most of these countries are also members of Nato, they don’t want unnecessary headquarters being established and further generals and staff being appointed.”

Former TV star Ksenia Sobchak has thrown her hat into next year’s presidential election. But even those who dislike Putin must concede he is a known quantity, says Mary Dejevsky, former foreign correspondent in Moscow, Paris and Washington